U.S. patent number 4,657,699 [Application Number 06/682,297] was granted by the patent office on 1987-04-14 for resistor compositions.
This patent grant is currently assigned to E. I. Du Pont de Nemours and Company. Invention is credited to Kumaran M. Nair.
United States Patent |
4,657,699 |
Nair |
April 14, 1987 |
Resistor compositions
Abstract
The invention is directed to a thick film resistor composition
for firing in a low oxygen-containing atmosphere comprising finely
divided particles of (a) a semiconductive material consisting
essentially of a refractory metal carbide, oxycarbide or mixtures
thereof and (b) a nonreducing glass having a softening point below
that of the semiconductive material dispersed in (c) organic medium
and to resistor elements made therefrom.
Inventors: |
Nair; Kumaran M. (East Amherst,
NY) |
Assignee: |
E. I. Du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
24739074 |
Appl.
No.: |
06/682,297 |
Filed: |
December 17, 1984 |
Current U.S.
Class: |
252/513; 29/620;
252/514; 338/308; 427/279; 501/32; 29/610.1; 252/512; 252/516;
427/226; 427/287 |
Current CPC
Class: |
H01C
17/0652 (20130101); Y10T 29/49082 (20150115); Y10T
29/49099 (20150115) |
Current International
Class: |
H01C
17/065 (20060101); H01C 17/06 (20060101); H01B
001/02 () |
Field of
Search: |
;252/516,518,512,519,513,514,520 ;338/308 ;29/61R,620
;427/226,279,287 ;501/32 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0008437 |
|
Mar 1980 |
|
EP |
|
0071190 |
|
Feb 1983 |
|
EP |
|
0146120 |
|
Jun 1985 |
|
EP |
|
58-36481 |
|
Aug 1983 |
|
JP |
|
Primary Examiner: Lechert, Jr.; Stephen J.
Claims
I claim:
1. A thick film resistor composition for firing in a low
oxygen-containing atmosphere comprising finely divied particles of
(a) a semiconductive material consisting essentially of a
refractory metal carbide, oxycarbide or mixture thereof, the
refractory metal being selected from Al, Zr, Hf, Ta, W, Mo and
mixtures thereof; and (b) a nonreducing glass having a softening
point below that of the semiconductive material, both (a) and (b)
being dispersed in (c) organic medium.
2. The composition of claim 1 which contains particles of a
conductive material selected from RuO.sub.2, Ru, Cu, Ni, Ni.sub.3 B
and mixtures and precursors thereof.
3. A method for making resistor elements comprising the sequential
steps of (a) printing upon a ceramic substrate a pattern of the
composition of claim 1; and (b) firing the composition in a low
oxygen-containing atmosphere to effect volatilization of the
organic medium therefrom and liquid phase sintering of the
glass.
4. The composition of claim 1 in which the semiconductive material
is selected from silicon carbide, silicon oxycarbide and mixtures
thereof.
5. The composition of claim 1 in which the nonreducing glass is
selected from alumino borosilicate glass containing Ca.sup.2+,
Ti.sup.4+ and Zr.sup.4+, alumino borosilicate glass containing
Ba.sup.2+, Ca.sup.2+, Zr.sup.4+, Mg.sup.2+, and Ti.sup.4+,
borosilicate glass containing Bi.sup.3+ and Li.sup.+, lead
germanate glass and mixtures thereof.
6. A resistor element comprising a thick film layer of finely
divided particles of a semiconductive material consisting
essentially of a refractory metal carbide, oxycarbide or mixture
thereof, the refractory metal being selected from Al, Zr, Hf, Ta,
W, Mo and mixtures thereof; and a sintered nonreducing glass having
a softening point below that of the semiconductive material, the
layer having been fired in a low oxygen-containing atmosphere to
effect liquid phase sintering of the glass.
Description
FIELD OF THE INVENTION
The invention relates to thick film resistor compositions and
especially those which are fireable in low oxygen-containing
atmospheres.
BACKGROUND OF THE INVENTION
Screen printable resistor compositions compatible with nitrogen (or
low oxygen partial pressure) fireable conductors are relatively new
in the art of thick film technology.
Thick film resistor composites generally comprise a mixture of
electrically conductive material finely dispersed in an insulative
glassy phase matrix. Resistor composites are then terminated to a
conductive film to permit the resultant resistor to be connected to
an appropriate electrical circuit.
The conductive materials are usually sintered particles of noble
metals. They have excellent electrical characteristics; however,
they are expensive. Therefore, it would be desirable to develop
circuits containing inexpensive conductive materials and compatible
resistors having a range of stable resistance values.
In general, nonnoble metal conductive phases such as Cu, Ni, Al,
etc. are prone to oxidation. During the thick film processing, they
continue to oxidize and increase the resistance value. However,
they are relatively stable if the processing can be carried out at
low oxygen partial pressure or "inert" atmosphere. As used herein,
low oxygen partial pressure is defined as the oxygen partial
pressure that is lower than the eqilibrium oxygen partial pressure
of the system consisting of the metal conductive phase and its
oxide at the firing temperature. Therefore, developed of compatible
resistor functional phases which are capable of withstanding firing
in a low oxygen partial pressure without degradation of properties
is the prime objective in this technology. The phases must be
thermodynamically stable after the processing of the resistor film
and noninteractive to the nonprecious metal terminations when they
are cofired in an "inert" or low oxygen partial pressure
atmosphere. The major stability factor is the temperature
coefficient of resistance (TCR). The materials are considered
stable when their resistance values do not change appreciably when
the resistor components are subjected to temperature changes.
BRIEF DESCRIPTION OF THE INVENTION
In its primary aspect, the invention is directed to a thick film
resistor composition for firing in a low oxygen-containing
atmosphere comprising finely divided particles of (a) a
semiconductive material consisting essentially of a refractory
metal carbide, oxycarbide or mixture thereof; and (b) a nonreducing
glass having a softening point below that of the semiconductive
material, dispersed in (c) organic medium.
In a second aspect, the invention is directed to a resistor element
comprising a printed layer of the above-described composition which
has been fired in a low oxygen-containing atmosphere to effect
volatilization of the organic medium and liquid phase sintering of
the glass.
PRIOR ART
Huang et al. in U.S. Pat. No. 3,394,087 discloses resistor
composition comprising a mixture of 50-95% wt. vitreous glass frit
and 50-5% wt. of a mixture of refractory metal nitride and
refractory metal particles. Disclosed are nitrides of Ti, Zr, Hf,
Va, Nb, Ta, Cr, Mo and W. The refractory metals include Ti, Zr, Hf,
Va, Nb, Ta, Cr, Mo and W. U.S. Pat. No. 3,503,801 Huang et al.
disclose a resistor composition comprising a vitreous glass frit
and fine particles of Group IV, V or VI metal borides such as
CrB.sub.2, ZrB.sub.2, MoBr.sub.2, TaB.sub.2 and TiB.sub.2. In U.S.
Pat. No. 4,039,997 to Huang et al. a resistor composition is
disclosed comprising 25-90 wt. % borosilicate glass and 75-10 wt. %
of a metal silicide. Disclosed metal silicides are WSi.sub.2,
MoSi.sub.2, VaSi.sub.2, TiSi.sub.2. ZrSi.sub.2, CaSi.sub.2 and
TaSi.sub.2. Boonstra et al. in U.S. Pat. No. 4,107,387 disclose a
resistor composition comprising a metal rhodate (Pb.sub.3 Rh.sub.7
O.sub.15 or Sr.sub.3 RhO.sub.15), glass binder and a metal oxide
TCR driver. The metal oxide corresponds to the formula Pb.sub.2
M.sub.2 O.sub.6-7, wherein M is Ru, Os or Ir. Hodge in U.S. Pat.
No. 4,137,519 discloses a resistor composition comprising a mixture
of finely divided particles of glass frit and W.sub.2 C.sub.3 and
WO.sub.3 with or without W metal. Shapiro et al. in U.S. Pat. No.
4,168,344 disclose resistor compositions comprising a mixture of
finely divided particles of glass frit and 20-60% wt. Ni, Fi and Co
in the respective proportions of 12-75/5-60/5-70% vol. Upon firing,
the metals form an alloy dispersed in the glass. Again, in U.S.
Pat. No. 4,205,298, Shapiro et al. disclose resistor compositions
comprising a mixture of vitreous glass frit having fine particles
of Ta.sub.2 N dispersed therein. Optionally the composition may
also contain fine particles of B, Ta, Si, ZrO.sub.2 and
MgZrO.sub.3. Merz et al. in U.S. Pat. No. 4,209,764 disclose a
resistor composition comprising a mixture of finely divided
particles of vitreous glass frit, Ta metal and up to 50% wt. Ti, B,
Ta.sub.2 O.sub.5, TiO.sub.2, BaO.sub.2, ZrO.sub.2, WO.sub.3,
Ta.sub.2 N, MoSi.sub.2 or MgSiO.sub.3. In U.S. Pat. No. 4,215,020,
to Wahlers et al. a resistor composition is disclosed comprising a
mixture of finely divided particles of SnO.sub.2, a primary
additive of oxides of Mn, Ni, Co or Zn and a secondary additive of
oxides of Ta, Nb, W or Ni. The Kamigaito et al. patent, U.S. Pat.
No. 4,384,989, is directed to a conductive ceramic composition
comprising BaTiO.sub.3, a doping element such as Sb, Ta or Bi and
an additive such as SiN, TiN, ZrN or SiC, to lower the resistivity
of the composition. Japanese patent application No. 58-36481 to
Hattori et al. is directed to a resistor composition comprising
Ni.sub.x Si.sub.y or Ta.sub.x Si.sub.y and any glass frit (" . . .
there is no specification regarding its composition or method of
preparation.").
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the invention are directed to heterogeneous
thick film compositions which are suitable for forming microcircuit
resistor components which are to undergo firing in a low
oxygen-containing atmosphere. As mentioned above, the low oxygen
atmosphere firing is necessitated by the tendency of base metal
conductive materials to be oxidized upon firing in air. The
resistor compositions of the invention therefore contain the
following three basic components: (1) one or more semiconductive
materials; (2) one or more metallic conductive materials or
precursors thereof; and (3) an insulative glass binder, all of
which are dispersed in (4) an organic medium.
The resistance values of the composition are adjusted by changing
the relative proportions of the semiconductive, conductive and
insulative phases present in the system. Supplemental inorganic
materials may be added to adjust the temperature coefficient of
resistance. After printing over alumina or similar ceramic
substrates and firing in low oxygen partial pressure atmosphere,
the resistor films provide a wide range of resistance values and
low temperature coefficient of resistance depending on the ratio of
the functional phases.
A. Semiconductive Material
The semiconductive materials which may be used in the compositions
of the invention are refractory metal carbides (MeC.sub.x),
oxycarbides (MeC.sub.y-x O.sub.x, where y=1-3 and x<1.) or
mixtures thereof. In particular, suitable refractory metals are Si,
Al, Zr, Hf, Ta, W and Mo. Of the refractory metals, Si is preferred
because silicon carbide is widely available in commercial
quantities.
Silicon carbide is a semiconductor with a large band gap of nearly
3 ev for hexagonal structure and 2.2 ev for the cubic modification.
Details are given in Proc. Int. Conf. Semiconductor Phys., Prague,
1960, 432, Academic Press, Inc. 1961 and Proc. Conf. Silicon
Carbide, Boston, 1959, 366, Pergamon Press, 1960. Small amounts of
impurities, which are always present in the commercial sample,
reduce the band gap. For example, if aluminum is the impurities,
the SiC is a p-type conducting with an acceptor level lying about
0.30 ev above the valance band; and if nitrogen is the impurity,
then the compound is n-type with a donor level lying about 0.08 ev
below the conduction band. Details are given in J. Phys. Chem.
Solids 24, 1963, 109 by H. J. Van Daal, W. F. Knippenberg and J. D.
Wasscher.
Refractory metal carbides, in general, have a range of solid
solubility, resulting in nonstiochiometric compositions with vacant
lattice sites (e.g., Ta, Ti, Mo, W, etc.). The range of the
solubility, structures, and phase compositions are summarized in
Aerojet-General Corporation Report on "Ternary Phase Equilibria in
Transition Metal-Boron-Carbon-Silicon System" dated Apr. 1, 1965.
Carbides are interstitial compounds and are structurally different
from their corresponding oxides. They always contain impurities
such as nitrides, oxides and free carbon.
Industrial scale manufacture of SiC by the Acheson Process is
described in various handbooks of chemical technology. The process
involves heating a mixture of silica and carbon in accordance with
a preselected temperature-time cycle. The major reactions that
takes place upon heating the mixture are as follows:
Also, there is evidence in the literature of the formation of SiO,
which further reduces to Si. It is considered that .alpha.-SiC is
an impurity-stabilized form of silicon carbide (R. C. Ellis; Proc.
Conf. Silicon Carbide, Boston, 1959, 124, Pergamon Press,
1960).
Fine powders of carbides and metal-doped carbides such as WC-6% Co
were prepared by reduction-carburization of metal oxide gels using
dry methane gas at 800.degree.-900.degree. C. The amorphous powder
thus obtained can be crystallized by heating in an oxygen-free
atmosphere at a higher temperature to obtain substantially pure
carbides. Alternatively, by heating the amorphous powder in a low
oxygen partial pressure atmosphere, oxycarbides are produced.
Details were described at the 79th Annual meeting of the American
Ceramic Society--Apr. 23-28, 1977, an abstract of which is given in
M. Hoch and K. M. Nair, Bulletin American Ceramic Soc., 56, 1977,
p. 289. Oxycarbides are also produced by heating a mixture of metal
carbide with the corresponding metal oxide in a controlled oxygen
atmosphere.
B. Glass Binder
The third major component present in the invention is one or more
of insulative phases. The glass frit can be of any composition
which has a melting temperature below that of the semiconductive
and/or conductive phases and which contains nonreducible inorganic
ions or inorganic ions reducible in a controlled manner. Preferred
compositions are alumino borosilicate glass containing Ca.sup.2+,
Ti.sup.4+, Zr.sup.4+ ; alumino borosilicate glass containing
Ca.sup.2+, Zn.sup.2+, Ba.sup.2+, Zr.sup.4+, Na.sup.+ ; borosilicate
glass containing Bi.sup.3+, and Pb.sup.2+ ; alumino borosilicate
glass containing Ba.sup.2+, Ca.sup.2+, Zr.sup.4+, Mg.sup.2+,
Ti.sup.4+ ; and lead germanate glass, etc. Mixtures of these
glasses can also be used.
During the firing of the thick film in a reducing atmosphere,
inorganic ions reduce to metals and disperse throughout the system
and become a conductive functional phase. Examples for such a
system are glasses containing metal oxides such as ZnO, SnO,
SnO.sub.2, etc. These inorganic oxides are nonreducible
thermodynamically in the nitrogen atmosphere. However, when the
"border line" oxides are buried or surrounded by carbon or
organics, the local reducing atmosphere developed during firing is
far below the oxygen partial pressure of the system. The reduced
metal is either evaporated and redeposited or finely dispersed
within the system. Since these fine metal powders are very active,
they interact with or diffuse into other oxides and form metal rich
phases.
The glasses are prepared by conventional glass making techniques,
by mixing the desired components in the desired proportions and
heating the mixture to form a melt. As is well known in the art,
heating is conducted to a peak temperature and for a time such that
the melt becomes entirely liquid and homogeneous. In the present
work the components are premixed by shaking in a polyethylene jar
with plastic balls and then melted in a crucible at up to
1200.degree. C., depending on the composition of the glass. The
melt is heated at a peak temperature for a period of 1-3 hours. The
melt is then poured into cold water. The maximum temperature of the
water during quenching is kept as low as possible by increasing the
volume of water to melt ratio. The crude frit after separation from
water is freed from residual water by drying in air or by
displacing the water by rinsing with methanol. The crude frit is
then ball milled for 3-5 hours in porcelain containers using
alumina balls. The slurry is dried and Y-milled for another 24-48
hours depending on the desired particle size and particle size
distribution in polyethylene lined metal jars using alumina
cylinders. Alumina picked up by the materials, if any, is not
within the observable limit as measured by X-ray diffraction
analysis.
After discharging the milled frit slurry from the mill, the excess
solvent is removed by decantation and the frit powder is then
screened through a 325 mesh screen at the end of each milling
process to remove any large particles.
The major properties of the frit are: it aids the liquid phase
sintering of the inorganic crystalline particulate matters; some
inorganic ions present in the frit reduce to conductive metal
particles during the firing at the reduced oxygen partial pressure;
and part of the glass frit form the insensitive functional phase of
the resistor.
C. Conductive Material
Because the semiconductive resistor materials generally have quite
high resistivities and/or highly negative HTCR (Hot Temperature
Coefficient of Resistance) values, it will normally be preferred to
include a conductive material in the composition. Addition of the
conductive materials increases conductivity; that is, lowers
resistivity and in some instances may change the HTCR value as
well. However, when lower HTCR values are needed, various TCR
drivers may be used. Preferred conductive materials for use in the
invention are RuO.sub.2, Ru, Cu, Ni, and Ni.sub.3 B. Other
compounds which are precursors of the metals under low oxygen
containing firing conditions can also be used. Alloys of the metals
are useful as well.
D. Organic Medium
The above-described inorganic particles are mixed with an inert
liquid medium (vehicle) by mechanical mixing (e.g., on a roll mill)
to form a pastelike composition having suitable consistency and
rheology for screen printing. The latter is printed as a "thick
film" on conventional ceramic substrates in the conventional
manner.
The main purpose of the organic medium is to serve as a vehicle for
dispersion of the finely divided solids of the composition in such
form that it can readily be applied to ceramic or other substrates.
Thus, the organic medium must first of all be one in which the
solids are dispersible with an adequate degree of stability.
Secondly, the rheological properties of the organic medium must be
such that they lend good application properties to the
dispersion.
Most thick film compositions are applied to a substrate by means of
screen printing. Therefore, they must have appropriate viscosity so
that they can be passed through the screen readily. In addition,
they should be thixotropic in order that they set up rapidly after
being screened, thereby giving good resolution. While the
rheological properties are of primary importance, the organic
medium is preferably formulated also to give appropriate
wettability of the solids and the substrate, good drying rate,
dried film strength sufficient to withstand rough handling, and
good firing properties. Satisfactory appearance of the fired
composition is also important.
In view of all these criteria, a wide variety of liquids can be
used as organic medium. The organic medium for most thick film
compositions is typically a solution of resin in a solvent
frequently also containing thixotropic agents and wetting agents.
The solvent usually boils within the range of
130.degree.-350.degree. C.
By far, the most frequently used resin for this purpose is ethyl
cellulose. However, resins such as ethylhydroxyethyl cellulose,
wood rosin, mixtures of ethyl cellulose and phenolic resins,
polymethacrylates of lower alcohols, and monobutyl ether of
ethylene glycol monoacetate can also be used.
Suitable solvents include kerosene, mineral spirits,
dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene
glycol, and high-boiling alcohols and alcohol esters. Various
combinations of these and other solvents are formulated to obtain
the desired viscosity and volatility.
Among the thixotropic agents which are commonly used are
hydrogenated castor oil and derivatives thereof and ethyl
cellulose. It is, of course, not always necessary to incorporate a
thixotropic agent since the solvent/resin properties coupled with
the shear thinning inherent in any suspension may alone be suitable
in this regard. Suitable wetting agents include phosphate esters
and soya lecithin.
The ratio of organic medium to solids in the paste dispersions can
vary considerably and depends upon the manner in which the
dispersion is to be applied and the kind of organic medium used.
Normally, to achieve good coverage, the dispersions will contain
complementally by weight 40-90% solids and 60-10% organic
medium.
The pastes are conveniently prepared on a three-roll mill. The
viscosity of the pastes is typically 20-150 Pa.s when measured at
room temperature on Brookfield viscometers at low, moderate and
high shear rates. The amount and type of organic medium (vehicle)
utilized is determined mainly by the final desired formulation
viscosity and print thickness.
Formulation and Application
The resistor material of the invention can be made by thoroughly
mixing together the glass frit, conductive phases and
semiconductive phases in the appropriate proportions. The mixing is
preferably carried out by either ball milling or ball milling
followed by Y-milling the ingredients in water (or an organic
liquid medium) and drying the slurry at 120.degree. C. overnight.
In certain cases, the mixing is followed by calcination of the
material at a higher temperature, preferably at up to 500.degree.
C., depending on the composition of the mixture. The calcined
materials are then milled to 0.5-2.mu. or less average particle
size. Such a heat treatment can be carried out either with a
mixture of conductive and semiconductive phases and then mixed with
appropriate amount of glass or semiconductive and insulative phases
and then mixed with conductive phases or with a mixture of all
functional phases. Heat treatment of the phases generally improves
the control of TCR. The selection of calcination temperature
depends on the melting temperature of the particular glass frit
used.
To terminate the resistor composition onto a substrate, the
termination material is applied first to the surface of a
substrate. The substrate is generally a body of sintered ceramic
material such as glass, porcelain, steatite, barium titanate,
alumina or the like. A substrate of Alsimag.RTM. alumina is
preferred. The termination material is then dried to remove the
organic vehicle and fired in a conventional furnace or a conveyor
belt furnace in an inert atmosphere, preferably N.sub.2 atmosphere.
The maximum firing temperature depends on the softening point of
the glass frit used in the termination composition. Usually this
temperature varies between 750.degree. C. to 1200.degree. C. When
the material cooled to room temperature, there is formed a
composite of glass having particles of conductive metals, such as
Cu, Ni, embedded in and dispersed throughout the glass layer.
To make a resistor with the material of the present invention, the
resistance material is applied in a uniform-drying thickness of
20-25.mu. on the surface of the ceramic body which has been fired
with the termination as described earlier. Compositions can be
printed either by using an automatic printer or a hand printer in
the conventional manner. Preferably the automatic screen printed
techniques are employed using a 200-325 mesh screen. The printed
pattern is then dried at below 200.degree. C., e.g. to about
150.degree. C. for about 5-15 minutes before firing. Firing to
effect sintering of the materials and to form a composite film is
preferably done in a belt furnace with a temperature profile that
will allow burnout of the organic matter at about
300.degree.-600.degree. C., a period of maximum temperature of
about 800.degree.-1000.degree. C. lasting about 5-30 minutes,
followed by a controlled cooldown cycle to prevent unwanted
chemical reactions at intermediate temperatures or substrate
fracture of stress development within the film which can occur from
too rapid cooldown. The overall firing procedure will preferably
extend over a period of about 1 hour with 20-25 minutes to reach
the firing temperature, about 10 minutes at the firing temperature,
and about 20-25 minutes in cooldown. The furnace atmosphere is kept
low in oxygen partial pressure by providing a continuous flow of
N.sub.2 gas through the furnace muffle. A positive pressure of gas
must be maintained throughout to avoid atmospheric air flow into
the furnace and thus an increase of oxygen partial pressure. As a
normal practice, the furnace is kept at 800.degree. C. and N.sub.2
or similar inert gas flow is always maintained. The above-described
pretermination of the resistor system can be replaced by post
termination, if necessary. In the case of post termination, the
resistors are printed and fired before terminating.
Test Procedures
In the Examples below, hot temperature coefficient of resistance
(HTCR) is measured in the following manner:
Samples to be tested for Temperature Coefficient of Resistance
(TCR) are prepared as follows:
A pattern of the resistor formulation to be tested is screen
printed upon each of ten coded Alsimag 614 1.times.1" ceramic
substrates and allowed to equilibrate at room temperature and then
dried at 150.degree. C. The mean thickness of each set of dried
films before firing must be 22-28 microns as measured by a Brush
Surfanalyzer. The dried and printed substrate is then fired for
about 60 minutes using a cycle of heating at 35.degree. C. per
minute to 850.degree. C., dwell at 850.degree. C. for 9 to 10
minutes and cooled at a rate of 30.degree. C. per minute to ambient
temperature.
Resistance Measurement and Calculations
The test substrates are mounted on terminal posts within a
controlled temperature chamber and electrically connected to a
digital ohm-meter. The temperature in the chamber is adjusted to
25.degree. C. and allowed to equilibrate, after which the
resistance of each substrate is measured and recorded.
The temperature of the chamber is then raised to 125.degree. C. and
allowed to equilibrate, after which the resistance of the substrate
is again measured and recorded.
The hot temperature coefficient of resistance (TCR) is calculated
as follows: ##EQU1##
The values of R.sub.25.degree. C. and Hot TCR are averaged and
R.sub.25.degree. C. values are normalized to 25 microns dry printed
thickness and resistivity is reported as ohms per square at 25
microns dry print thickness. Normalization of the multiple test
values is calculated with the following relationship: ##EQU2##
Coefficient of Variance
The coefficient of variance (CV) is a function of the average and
individual resistances for the resistors tested and is represented
by the relationship .sigma./R.sub.av, wherein ##EQU3## R.sub.i
=measured resistance of individual sample. R.sub.av =calculated
average resistance of all samples (.SIGMA..sub.i R.sub.i /n)
n=number of samples
CV=(.sigma./R).times.100(%)
The invention will be better understood by reference to the
following examples in which all compositions are given in
percentages by weight unless otherwise noted.
EXAMPLES
In the Examples which follow, the following glass composition was
used:
TABLE 1 ______________________________________ Glass Frit
Compositions A B ______________________________________ CaO 4.0%
wt. -- ZnO 27.6 -- SiO.sub.2 21.7 3.5 B.sub.2 O.sub.3 26.7 3.5
Na.sub.2 O 8.7 -- Al.sub.2 O.sub.3 5.7 -- ZrO.sub.2 4.0 -- BaO 0.9
-- PbO 0.7 11.0 Bi.sub.2 O.sub.3 -- 82.0
______________________________________
EXAMPLES 1-4
Using the formulation and testing procedures described above, a
series of three resistor compositions was prepared in which various
concentrations of SiC, a semiconductor, were used as the conductive
phase in combination with Glass A. Furthermore, in Example 4, a
small amount of AlOOH, a TCR driver, was substituted for part of
the SiC as in the composition of Example 1. The composition of the
formulations and the electrical properties of the resistors
prepared therefrom are given in Table 2 below. The resistor data
show that as SiC is used to replace glass, the very high resistance
values are lowered only slightly and that the quite highly negative
HTCR values become even more highly negative. In addition, it can
be seen that the AlOOH functioned as a positive TCR driver in that
the HTCR of Example 4 was considerably less negative than that of
Example 1.
TABLE 2 ______________________________________ Effect of
Semiconductor Concentration on Resistor Properties Example No. 1 2
3 4 (% wt.) ______________________________________ Composition SiC
50 40 30 40 Glass A 20 30 40 20 AlOOH -- -- -- 10 Organic Medium 30
30 30 30 Resistor Properties R, .OMEGA./.quadrature. 3.60 .times.
3.99 .times. 4.94 .times. 8.40 .times. 10.sup.6 10.sup.6 10.sup.6
10.sup.6 HTCR, ppm/.degree.C. -10,947 -9,008 -5,614 -6,600
______________________________________
EXAMPLES 5-7
Again using the formulation and testing procedures described above,
a series of three additional resistor compositions was prepared in
which an organosilane ester was used to replace a progressively
greater amount of the semiconductor. The organosilane ester readily
decomposes during firing to form (SiO.sub.4).sup.4- tetrahedra
which reacts with components of the glass binder.
The compositions of the formulations and the electrical properties
of the resistors prepared therefrom are given in Table 3 below.
These data show the inclusion of the silicon ester to replace part
of the SiC resulted in slightly lower HTCR values, but the
composition still had high resistance values.
TABLE 3 ______________________________________ Effect of Silane
Ester Addition Example No. 5 6 7 (% wt.)
______________________________________ Composition SiC 30 20 10
AlOOH 10 10 10 Silane ester 10 20 30 Glass A 20 20 20 Organic
Medium 30 30 30 Resistor Properties R, .OMEGA./.quadrature. 3.54
.times. 10.sup.6 22.54 .times. 10.sup.6 8.01 .times. 10.sup.6 HTCR,
ppm/.degree.C. -8,250 -6,380 -5,830
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EXAMPLES 8-10
A further series of three resistor compositions was formulated in
which Ni.sub.3 B, a conductor, was added to the semiconductive SiC.
The formulation also contained a small but constant amount of
Al.sub.2 O.sub.3. The composition of the formulation and the
electrical properties of the resistors prepared therefrom are given
in Table 4 below.
Because Ni.sub.3 B is a conductor and SiC is only semiconductive,
one would expect that the replacement of SiC with Ni.sub.3 B would
result in significant lowering of the resistance values of the
composition. However, quite surprisingly, this did not happen, for
the resistance values of the composition were only slightly
changed. The values of HTCR were little changed as well.
TABLE 4 ______________________________________ Effect of Ni.sub.3 B
Addition Example No. 8 9 10 (% wt.)
______________________________________ Composition SiC 15 10 5
Ni.sub.3 B 5 10 15 Al.sub.2 O.sub.3 5 5 5 Glass B 25 25 25 Organic
Medium 50 50 50 Resistor Properties R, .OMEGA./.quadrature. 40.8
.times. 10.sup.3 26.2 .times. 10.sup.3 35.1 .times. 10.sup.3 HTCR,
ppm/.degree.C. -6,907 -8,850 -6,900
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